<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<form action="el.jsp" method="post">
    username:<input type="text" name="username" value="<%= request.getParameter("username") %>"/>
    <br/>
    username:<input type="text" name="username" value="${param.username}"/>
    <input type="submit" value="提交"/>
</form>

username:<%= request.getParameter("username") %>
</body>
</html>
